Conventionally, to transmit and receive data using a network such as the Internet, connection to a communication counterpart terminal apparatus is established through a LAN (Local Area Network) or a WAN (Wide Area Network) that is configured according to any one of various standards such as Ethernet (a registered trade mark), an FDDI (Fiber-Distributed Data Interface), and an ATM (Asynchronous Transfer Mode).
A given relay apparatus such as a router is equipped between networks. The relay apparatus has a function of monitoring whether transmission data transmitted to a network is transmittable. Therefore, when the relay apparatus finds an error in the transmission data, the relay apparatus transmits an error message corresponding to the content of the error to a communication terminal apparatus that is the transmission source of the transmission data.
An error message may be, for example, a packet too big message, a destination unreachable message, a time exceeded message, and a parameter problem message. The details of processing for using a network based, in particular, on the IPv6 (Internet Protocol Version 6) will be described taking an example where a packet-capacity exceeded message is transmitted from a relay apparatus.
(MTU of Network)
As described, to transmit and receive data between given terminal apparatuses using the Internet, multiple networks must be passed through using relay apparatuses. The networks each have set therein a permissible data size that is transmittable for one packet of transmission. This permissible amount is generally referred to as “MTU (Maximum Transmission Unit)”. When multiple networks are passed through to transmit and receive data between communication terminal apparatuses, a terminal apparatus for transmission must adjust the capacity of transmission data such that the transmission data may be transferred through the network having the smallest MTU.
FIG. 10 is a diagram for explaining a monitoring process of transmission data by a relay apparatus. The relay apparatus not only relays between networks but also monitors the capacity of transmission data such that a terminal apparatus for transmission may transmit transmission data of a capacity corresponding to the MTU of a network.
As depicted in FIG. 10, two networks, a network A and a network B, are configured between two communication terminal apparatuses 1000 and 1010. Relay apparatuses R1 and R2, and R3 and R4 are respectively disposed in the networks A and B. Explanation will be given assuming that the communication terminal apparatus 1010 is a terminal for transmission, the communication terminal apparatus 1000 is a terminal for reception, and the MTU of the networks A and B is 1,500 [bytes].
To transmit transmission data from the communication terminal apparatus 1010, a case is tentatively assumed first where the transmission data is output from an application unit 1011 to communication processing interfaces (I/Fs) 1012A and 1012B (S10). A process of actually outputting packets using application software simultaneously to two interfaces is not common. However, to describe the relation between the transmission data and the MTU, a case where the transmission data is output to the communication processing interfaces (I/Fs) 1012A and 1012B is temporarily assumed and will be described for convenience.
In this example, transmission data of a capacity of 1,500 [bytes] or less, representing packet size, is output to the communication processing interface (I/F) 1012A and transmission data of a packet size of 1,500 [bytes] or larger is output to the communication processing interface (I/F) 1012B.
The transmission data transmitted from the communication processing interface (I/F) 1012A to the network A has a capacity equal to or less than the MTU and therefore, the transmission data is received by the communication processing interface (I/F) 1001A of the communication terminal apparatus 1000 for reception through the relay apparatuses R2 and R1 (S11).
On the other hand, the transmission data transmitted from the communication processing interface (I/F) 1012B to the network B has a capacity exceeding the MTU and the relay apparatus R3 detects that the capacity is larger than the MTU (S12). Hence, the transmission data is not transmitted to the communication processing interface (I/F) 1001B of the communication terminal apparatus 1000 and instead, an error message “Packet Too Big” indicating that the transmission data exceeds the MTU is transmitted to the communication terminal apparatus 1010 (S13).
When the communication processing interface (I/F) 1012B of the communication terminal apparatus 1010 receives the error message “Packet Too Big” from the network B, the interface (I/F) 1012B reduces the packet size of the transmission data. This series of processing is known as “Path MTU Discovery (RFC 1981)” (see, for example, Non-Patent Literature 1).
Recently, transmission and reception of data using a network that uses UDLR (Uni-Directional Link Routing) has also been realized. FIG. 11 is a diagram for explaining a monitoring process of a relay apparatus in a network using the UDLR.
As depicted in FIG. 11, the network using the UDLR includes a distribution apparatus 1100 that distributes data (distribution data) using a unidirectional communication network, and a communication terminal apparatus 1110 that receives the transmission data. The distribution apparatus 1100 and the communication terminal apparatus 1110 are connected to a unidirectional communication network C dedicated for distribution, and a bidirectional communication network D (a normal network) used for transmitting data from the communication terminal apparatus 1110 to the distribution apparatus 1100.
When the distribution data is distributed from the distribution apparatus 1100 to the communication terminal apparatus 1110, the distribution data is output from a distributing interface (I/F) 1101 to the unidirectional communication network C (S00). From the distribution apparatus 1100, the distribution data may be distributed to a specific communication terminal apparatus or may be distributed to multiple communication terminal apparatuses as a broadcast wave.
The communication terminal apparatus 1110 receives the distribution data using a receiving interface (I/F) 1112. In a network using the UDLR, to transmit a response to the distribution apparatus 1100, transmission data is output from an OS/application unit 1111 to the receiving interface (I/F) 1112 that is connected to the unidirectional communication network C (S20). To transmit as a response to the distribution apparatus 1100, the transmission data that is transmitted as a response to the unidirectional communication network C, the receiving interface (I/F) 1112 realizes a virtual bidirectional communication network by transmitting the transmission data to the bidirectional communication network D. Therefore, the receiving interface (I/F) 1112 applies a given process to the transmission data and outputs the processed transmission data to a communication processing interface (I/F) 1113 (S21).
The communication processing interface (I/F) 1113 transmits the transmission data to the bidirectional communication network D. Here, when the transmission data has a capacity equal to or less than the MTU, the transmission data is transmitted to the distribution apparatus 1100 through the relay apparatuses R3 and R4.
Nonpatent Literature 1: Izumi, Miki, “Serial Article: Invitation to Ipv6 Networks (No. 3) ‘Header Format of Ipv6’”, [online],Oct. 5, 2001, Research Institute for Internet Strategies, Inc., [Retrieved on Jul. 7, 2006].